This invention relates to a device useful for determining the degree of dispersion of particles in a particle/vehicle system, such as liquid coatings and their intermediates. The invention relates more particularly to a machine combination that is especially well-suited for determining the degree of dispersion of particulate materials, including pigments, extenders, and fillers used in paints and coatings, and methods for using the combination.
Various properties of a coating are influenced to a large extent by the size of the particulate materials present at the coating surface, including the opacity, color, shade, tint, gloss, and weatherability. In order to achieve optimized physical and performance properties from a coating, it is generally found that there exists an optimum range of sizes for the particles for the particular intended use or application under consideration. Thus, for particulate materials which are intended for use in paints, it is necessary to cause the particles to possess a maximum particle size. This is most frequently achieved by using a mill or otherwise grinding the pigment to a relatively uniform grind size prior to its dispersion into a vehicle, such as a paint formulation, to achieve the desired maximum size and in particular to break up particle agglomerations. Since it is generally undesirable from the standpoint of capital resources to employ excessive milling when grinding the particles, it has been found beneficial by those in the industry to employ precise and accurate test methods for determining the size distribution to which the particles have been ground or milled. Such tests are generally referred to as xe2x80x9cfineness of grindxe2x80x9d tests, and these typically give a measure of the maximum size present in the sample tested.
By far the most popular test method which is used in the art is set forth in the specification of ASTM D-1210. The main hardware component used in this Standard test is a solid rectangular gauged block having a flat top surface and which incorporates either a single shallow channel or two parallel shallow channels machined into this surface. Each channel is tapered uniformly in depth along its length dimension from a depth of zero at one end, to a specified depth, e.g. 50 or 100 xcexcm, at the other end. There are calibration marks disposed along the length of the channel(s) at graduated depths so as to form a scale, wherein each mark corresponds to the depth at that point. Often, such gauges are calibrated in terms of Hegmann units, a scale that is well-known to those skilled in the art of particle size determination. In use, a sample to be tested is placed at the deep end of a channel of increasing depth as per the above so that it overflows the walls of the channel slightly, and is subsequently drawn down the path towards the shallow end using a flat scraper blade of suitable dimension. The gauged block containing sample in the channel is then disposed between the operator and a light source, and observation is made of the point along the scale at which a definite speckled pattern (as distinct from isolated specks) first appears. Location of this point gives a measure of the degree of the fineness of the grind. In addition the point at which xe2x80x9cstragglersxe2x80x9d, i.e., isolated particles as distinct from a definite speckled pattern, first appear can be assessed. The sample can then be assigned a grading based on the observation of where these points lie on the scale.
However, this type of testing has its drawbacks. First, such test methods are subjective, as the assessment result obtained depends on the personal observation and judgement by a human operator as to where along the scale the above-mentioned features lie. Since the point on the scale at which these features occur is not sharply defined, there exists some latitude for personal interpretations and biases of the observer to influence the final results obtained, and it is quite common for different operators to record different results for the same sample of material. Secondly, it is important that the assessment by the observer be carried out before the sample dries to any appreciable extent, which is within 10 seconds according to the Standard. The reason that a quick observation is necessary is that the thickness of sample films generally decrease during drying. The 10-second limit often makes assessment by the operator a difficult task. Thirdly, once the test has been completed and the gauge cleaned prior to a subsequent determination, no physical record remains other than the assessment figures recorded by the operator.
Thus, if a reasonably-priced method or device were available by which operator bias could be substantially reduced or eliminated and which would produce a permanent record of a particle size determination for any specific sample of particulate material, such method or device should be welcomed by the industry.
U.S. Pat. No. 5,249,029, which is fully incorporated herein by reference thereto, discloses an apparatus that meets these requirements, which apparatus embodies the features of: a) a light source with a focussing lens for producing a light spot on the paint surface to be investigated; b) a dark-field lens comprising an objective for collecting the scattered light emanating from particles illuminated by the light spot and an aperture diaphragm for eliminating the light directly reflected at the paint surface; c) a photoreceiver connected to an evaluation circuit for detecting, recording and further processing the scattered light signals; and d) a scanner for longitudinally scanning the sample channel. The scanner consists of a synchronous motor by which the grindometer block is longitudinally displaced on a carriage linearly as a function of time, the light spot moving over the paint surface to be investigated. According to a preferred embodiment, the scattered light signals are counted by the evaluation circuit in zones along the paint surface during scanning. To this end, a narrow slot laterally disposed to the sample channel is provided in front of the detector and improves zone resolution. This invention of the prior art alleges the following advantages: 1) application and stripping of the paint surface on the grindometer block is followed by fully automatic, objective measurement; 2) measurement of the particle size distribution accords very well with the conventional visual evaluation; 3) the reproducibility of the apparatus is sufficient for all purposes; and 4) measurement of the particle size distribution is possible for all the usual colors.
However, the invention disclosed in U.S. Pat. No. 5,249,029 is not without inherent disadvantages. For example, the apparatus described therein is confined to the use of a laser source to provide the required virtual point source. Further, the apparatus described therein requires a focusing lens to direct the laser beam onto the paint surface. This apparatus also requires receiving optics that provide for the exclusion of ambient light by means of a narrow-band, 633 nm filter, dark-field optics incorporating a central beam block to exclude specular reflected laser light, and a slot to improve data resolution in the scan direction. This apparatus further employs a single silicon photodetector responding to the average level of scattered light across the full width of the paint channel in the grindometer block. Thus, it cannot distinguish individual particles protruding through the surface of the paint film where two or more particles are arranged on a transverse line. Finally, the apparatus of U.S. Pat. No. 5,249,029 does not read the standard scale marks on the grindometer gauge.
The present invention comprises a combination useful for assessing the fineness of dispersion of particles in a liquid vehicle, and comprises: a light source; a moveable carrier having a top surface; a grindometer gauge block disposed on the top surface of the carrier. The block has a longest length dimension and a flat face surface, and comprises a linear channel disposed on its face that varies in depth along the length dimension and has corresponding markings to indicate the depth of the channel. The channel is adapted to receive a liquid vehicle which contains a film-forming dispersion of particles. There is an image-producing means, useful for producing a two-dimensional electronic image of a film disposed in said channel from a liquid vehicle placed therein, and a memory means for archiving said two-dimensional electronic image of the film. There is also a means for displaying said two-dimensional image of said film. The light source, grindometer gauge block, and image-producing means are sufficiently positioned so that the view of the image-producing means is aimed into the dark field below the light source so as to render the image-producing means to be disposed so that the light it receives is predominantly that which is reflected from discontinuities in the surface of said film. In this context, xe2x80x9cpredominantlyxe2x80x9d means at least 95% of the light on the basis of total energy, for the purposes of this specification and the appended claims.